Over two months towards the end of last year, several organizations began detecting unusual levels of radioactivity billowing across much of Europe. The particles – which were rapidly assessed to be harmless to people – were tentatively traced to somewhere around the Ural Mountains, although rather worryingly no one was entirely sure of their origin, and no one was admitting to the release.

Now, a new report published this month by the French Institute of Radioprotective and Nuclear Security (IRSN), who have been trying to piece together what led to the discharge of ruthenium-106, a radioactive nucleotide that is created by splitting atoms in a reactor, and does not occur naturally, appears to come to a fairly solid conclusion.

They say that in all likelihood it was the mishandling of spent nuclear fuel in a Russian facility that accidentally ejected the particles. Not everyone agrees with this, however, not least the Russians.

Using the weather patterns and air currents that flowed across Europe during September and October 2017, coupled with readings from various institutions, it was not long after the mysterious nuclear accident took place that the IRSN was able to follow the radioactivity back to its general origin.

As early as November they were able to say with a decent degree of accuracy that the event likely took place somewhere near the border between Russia and Kazakhstan in the Ural Mountains. This new report closes in ever further, however, and suggests that the isotopes came from a Russian nuclear facility in the region called the Mayak Production Association.

This gives the where, but the IRSN even think they have a pretty good guess at the why, too. They suspect that the leak occurred after the Mayak facility botched the production of highly radioactive components needed for a physics experiment that was due to take place at an Italian laboratory at the beginning of this year, an experiment that has subsequently been called off.

They think that in 2016 the Mayak facility was commissioned to make a capsule of cerium-144 for use by the Gran Sasso National Laboratory in L’Aquila, Italy, in the hunt for a theoretical particle known as a sterile neutrino. The experiment requires highly radioactive material, and it's thought that Mayak was trying to achieve this by processing several tons of spent nuclear fuel just a year or two after it was removed from a nuclear reactor.

It is suggested that during this processing, some of the ruthenium present in the material was converted into ruthenium oxide, which passed through the facility's filters and was ejected into the atmosphere, before spreading across Europe last October. The facility subsequently stopped all production, fitting with the cancellation of the Italian experiment in December 2017.

While this narrative all seems to fit, the Russians say it is inaccurate. They maintain it was a "rare meteorological event".